Electrical wiring system with overtemperature protection

ABSTRACT

A household electrical service (10) is protected against overheating by running a sensor line or wire (36 or 38), coextensively together with hot, neutral and ground lines or wires (12, 13, 14 or 31, 32, 33), from a power company hookup (50) to a main breaker switch (18), and from respective branch circuit breakers (19) throughout the branch circuits (21, 22). The sensor line or wire (36 or 38) may be a length of heat fusible or otherwise temperature variant material which has a load end connected to ground and a source end connected to a control circuit (51) which trips the breaker (18 or 19) when an overtemperature condition melts the wire. Protection is extended through to individual devices (43) by continuing the sensor wire through the device cord (42). For this purpose, a plug ground prong (46) and receptacle ground prong connection (45) provide isolated ground and sensor wire connections. In a modified installation, fiber optic paths (38&#39;) are used in place of conductive wires and LED circuits replace load end ground connection.

This invention relates, in general, to electrical wiring systems; and,in particular, to a system such as a household electrical wiring systemincluding a temperature sensing mechanism for detecting anovertemperature condition and automatically interrupting an electricalcircuit in response thereto.

BACKGROUND OF THE INVENTION

A typical electrical service for a modern household includes a minimum150-amp. 220 V. installation comprising a service head hookup fromutility company power lines, a usage meter, an entrance service panel(also referred to a "fuse box") including protective devices such asfuses and/or circuit breakers, and multiple branch circuits fordistributing electrical power throughout the house to appliances,lighting and wall outlets. The circuits characteristically employ one ormore "hot" wires, a "neutral" wire and usually a "ground" wire combinedin a single sheathing. All wires, except for the ground wire, arenormally individually insulated to keep them from touching and causing ashort.

One of the most common wires used in household electrical systems istype T wire which is wrapped in thermal plastic insulation. The usualcabling is NM or nonmetallic sheath cable (commonly called "Romex")comprising two or more individually insulated type T wires and a barecopper grounding wire, all coated with a plastic sheath. The wires maybe directly embedded in the cable itself (type NMC or UF cable) or havespace filled with jute (type NM cable). Household circuits may alsoemploy armored cable (commonly called "BX") wherein the wires arewrapped in heavy paper and surrounded by a spiral sheath of flexiblesteel or aluminum. Armored cable is rarely found in newly constructedhomes.

Protection for current overloads and short circuits is conventionallyprovided by fuses and circuit breakers. Fuses generally compriseelongated metal wires through which the electrical current is caused toflow and which melt ("blow") when too much current is applied. Thisresults in a gap or "open circuit" through which electricity can nolonger flow, thus cutting off power. The fuse is designed to blow if theelectricity requirements of the appliances and other electrical devicesplugged into a particular circuit exceed the amperage rating of thatcircuit. The fuses will also blow if wires directly touch one anothercausing a short circuit. Circuit breakers are switches that actsimilarly to fuses for protecting circuits. However, instead ofpermanently severing a wire when an overload occurs, the breaker is"tripped" to switch from an "on" to an "off" position. Once the overloadproblem has been corrected, the breaker can simply be reset by switchingit back to the "on" position.

While fuses and circuit breakers both provide protection for currentoverloads and short circuits, neither provides protection against the"partial short circuit." This condition exists when a current load thatdoes not reach the overload rating of the circuit's conventionalprotection flows through the wires, but nevertheless causes excessiveheat sufficient to present a risk of fire. Three examples of occurrencesof this problem are as follows:

1. A mouse chewed off properly rated, nonmetallic-sheathed "Romex" cablein an attic of a house. A hot electrical wire partially shorted to theground causing a fire. Fortunately, the wire melted apart and terminatedthe current flow, before any serious damage occurred. Electricity to theload side was interrupted, but the circuit breaker did not trip.

2. An air conditioner compressor fan switch mounted on the side of ahouse corroded causing the outside unit to not function. Upon throwingthe switch to check for proper operation, the switch's internalcomponents shorted causing a fire on the side of the house. The firecontinued until the 60-amp. circuit breaker in the garage was tripped.Upon closer examination, it was observed that the wires going to theoutside switch melted off the switch and continued to stay shorted asinsulation melted away.

3. A telephone installer drilled a hole in the side of a house toinstall a jack. The main electrical service for the house was on theother side of the wall. The drill bit shorted the main feed to the homeservice panel box inside the house. The box became so hot that it setfire to the surrounding paneling. Aerial wire to the house became so hotthat it broke away from the house and continued to stay shorted settingfire to the yard.

A ground fault circuit interrupter (GFCI) provides protection againstshock due to a "ground fault." A ground fault can occur when a bare hotwire touches a grounded wire, an armored cable, a metal conduit or thecurrent is otherwise redirected from its normal path to ground. The GFCImonitors the equal current in the "hot" and "neutral" wires, and shutsoff all current if there is a drop in the return current. Receptacleversions of GFCI devices, if installed on a first receptacle of acircuit, will usually also protect against ground faults that mightoccur at other receptacles or devices in the same circuit. GFCIs arealso available that are installed directly in the breaker panel andcombine a breaker switch with a GFCI. Such combination protects againstoverloads, shorts and ground faults. The partial short circuit discussedabove, which overheats a wire and may cause a fire, may not result in aninequality between outgoing and return currents, so may not trip theGFCI.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a mechanism for protectingan electrical wiring system, such as a modern household electricalservice, against overheating which does not cause a current capacityoverload, short circuit or ground fault against which conventionalprotective devices protect.

In accordance with the invention, an electrical wire system is augmentedby running a sensor wire, along with hot, neutral and ground wires,within the cable throughout the electrical service. One end of thesensor wire in each hookup and branch service path is connected to asensor monitoring circuit, and the other end is either directly orthrough the cord of each electrical device connected to ground. Changesin the sensor wire caused by overtemperature applied to the cable, aredetected at the sensor monitoring circuit and a circuit breaker, GFCI,or other circuit interrupting element is tripped in response thereto, tobreak the current.

In the first embodiment, described in greater detail below, the sensorwire may take the form of a lead or lead fabric wire run within theusual electrical cabling throughout an electrical wiring system. Thelead wire is attached to ground at the load end of each branch circuitand to a sensor input to a circuit breaker on the source end. Should thewire become subject to overtemperature at any point along its length, itmelts and activates a control circuit in the breaker that trips thecircuit.

An alternative embodiment, also discussed below, utilizes a temperaturesensitive fiber optic lead in place of the electrically conductive leadwire. An LED light source is placed at the load side and the sensorsystem is placed at the source side. As the temperature of the fiberoptic wire increases, the light path changes from translucent to dark,thus stopping the light flow which trips the circuit. In addition toprotecting the branch circuits, a sensor wire is also placed along thefeed from the power company, so that any temperature increase betweenthe outside transformer and the meter box, or the meter box and theservice panel, will shut down electrical service to the structure.

For extending the same protection to the cords of electrical appliancesthat plug into receptacles, a "duplex" style plug may be provided with aspecial "split" ground prong, to assure sensing overtemperatureprotection all the way to the actual load.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention have been chosen for purposes ofillustration and description, and are shown in the accompanying drawingswherein:

FIG. 1 is an overall view of a household electrical wiring systemincluding an overtemperature protective mechanism in accordance with theinvention;

FIG. 2 is a fragmentary perspective view of a nonmetallic sheath cableusable in the electrical system of FIG. 1;

FIG. 3 is a view of a receptacle wired for the extension ofovertemperature protection all the way to the plugged in electricaldevice;

FIG. 4 is a view of an electrical device plug and cord usable with thereceptacle of FIG. 3;

FIG. 5 is a block diagram of branch circuit incorporatingovertemperature protection; and

FIG. 6 shows the wiring for an electrical device protected with anoptical sensor wire connection.

Throughout the drawings, like elements are referred to by like numerals.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As shown in FIG. 1, a modern household electrical service includes anelevated service head 10 having an entry port 11 into which hot wire,neutral wire and ground wire lines 12, 13, 14 are directed from thepower company pole-top step-down transformer via vertical conduit 15 toa standard usage meter 17. From meter 17, the same lines 12, 13, 14 arethen wired through a breaker type, main entrance switch 18 to the inputterminal of a bank of breakers and/or fuses 19 installed in a servicepanel fuse box 20. Each breaker or fuse 19 is wired to protect adifferent branch circuit 21, 22 which distributes electrical power inthe house to a different one or more appliances, lighting fixtures, walloutlet receptacles, or the like. For illustrative purposes, FIG. 1 showsa first branch circuit 21 wired to provide power to a wall receptacle24; and a second branch circuit 22 wired to provide power under controlof a toggle switch 25 to a lighting fixture 26. Connections between thebreaker or fuse 19 and the electrical elements 24, 25, 26 is made usingnonmetallic sheath cabling 28 (FIG. 2) comprising hot, neutral andground wires 31, 32, 33 embedded in a plastic insulative sheath 34.Connection of wires 31, 32, 33 are made in conventional manner. For theillustrated wiring, wire 31 is a black wire (with wire 31' being a whitecoded black wire), wire 32 is a white wire and wire 33 is a green orunjacketed copper ground wire.

In accordance with the first implementation of the invention, a sensorwire line 36 is run coextensively with the lines 12, 13, 14 between thestep-down transformer and main entrance switch 18, and also between mainentrance switch 18 and service panel 20. A sensor wire 38 is similarlyrun coextensively with wires 31, 32, 33 within nonmetallic sheathcabling 28 (FIG. 2) from service panel 20 throughout each branch circuit21, 22. As with wires 31, 32, 33, sensor wire 38 may be directlyembedded in plastic sheath 34.

For each run of electrical service to be protected, a source end of line36 or wire 38 is connected to a control circuit of a breaker or othercircuit interrupting device and a load end of the same line 36 or wire38 is connected to ground line 14 or ground wire 33. Thus, as shown inFIG. 1, the source end of a first run of line 36 connects to a powerinterrupting control circuit at the output to the step-down transformerand the load end of the same first run connects to ground line 14 at themetal housing of main entrance switch 18. A second run of line 36 hasits source end connected to a control circuit wired to control trippingof main breaker 18, and its load end connected to ground line 14 at themetal housing of service panel 20. For protection of each branch circuit21, 22, a run of sensor wire 38 has a source end connected to a controlcircuit wired to trip a respective branch circuit breaker 19, and a loadend connected to ground 33 at the last electrical element of the branch.Thus, in the illustrated example, sensor wire 38 is shown passingthrough the octagon box 39 of middle-of-the-run lighting fixture 26 andconnected to ground wire 33 at rectangular Gem box 40 of switch 25.Similarly, sensor wire 38 of branch circuit 21 is shown connected toground wire 33 at rectangular Gem box 41 of end-of-the-run receptacle24.

The sensor lines 36 and sensor wires 38 may comprise lengths of fusible,conductive material with predetermined low melting point, such as leador lead fabric. The lengths are insulated from surrounding material andother lines or wires, except at their end connections. The source endsare connected to circuitry formulated in accordance with knowntechniques to interrupt power at the power company transformer, mainbreaker switch 18 or respective breaker 19, whenever the correspondingload end connection to ground is lost.

As shown in FIGS. 3 and 4, protection can be extended right through tothe cords of electrical appliances. This is accomplished by withholdingthe connection of sensor wire 38 to ground wire 33, until thetermination of electrical device cord 42 within the device 43 itself.This is particularly useful for heavy duty appliances, and especiallyones for which cord 42 is either partially concealed or runs in aconfined space. One method of accomplishing this is to provide amodified wall receptacle 24' (FIG. 3) wherein the female ground prongconnection 45 is split into electrically isolated left and right halvesfor receiving corresponding electrically isolated left and right halvesof a plug male ground prong 46 (FIG. 4). One half of female connection45 is connected to ground wire 33 and the other half is connected tosensor wire 38. Likewise, one half of male prong 46 is connected to cordwire 33 and the other half is connected to cord sensor wire 38. Thehalves are separated by insulating material 48 at both receptacle 24'and plug 49, so that wires 33, 38 remain unconnected until the load endof cord 42. The connection between wires 33, 38 is then made within thegrounded housing of device 43.

A block diagram of the overtemperature protective system of electricalbranch circuit 21 is shown in FIG. 5. Power source 50 corresponds to thepower company transformer and switch 54 corresponds to the particularservice panel breaker 19 associated with branch circuit 21. Sensorcircuit 51 acts to trip switch 19 to terminate power to wires 31, 32whenever the connection of line 38 to ground is broken. This occurs whenthe temperature in the line pathways or cabling exceeds the meltingpoint of the fusible material.

Circuit 51 and switch 19 may be combined into a single combined unitthat fits into service panel 20, similar to GFCIs that combine a breakerwith a circuit interrupter that installs directly into the breakerpanel. As with the GFCI, alternative arrangements of circuit 51 are alsopossible which provide circuit interruption by placing a circuit 51built into a receptacle. As with such built-in GFCI outlets, a circuit51 installed in this manner on a first receptacle of a branch circuit21, 22 will also protect against overtemperatures that might occuranywhere along the same branch circuit. For local protection of aparticular device 43, a plug-in version of circuit 51 that fits into anythree-slot outlet is also possible. Provision can also be made tocombine the installations of receptacle 24 (FIG. 1) and 24' (FIG. 3),into a hybrid version that will connect sensor wire 38 to ground wire 33at the receptacle when no plug 48 is present, but will withhold theconnection until device 43 when plug 49 is present. This can beaccomplished, for example, by providing a spring-loaded conductornormally joining the halves of female connection 45, but which isdisplaced to break the connection when male prong 46 is inserted.

FIG. 6 illustrates a modified installation wherein an optical fibersensor line or wire is used in place of a fusible conductor line orwire. Instead of connecting the load end of the sensor line or wire toground, the load end is provided with an LED or similar light sourcecircuit 53 which directs a light beam into an optical fiber wire 38'installed within cabling 28'. In this case, the sensor circuit 51 ismodified to include light receptive transducer means for monitoring thestrength of illumination. The optical path 38' is formulated to changeits light transmission characteristics when exposed to heat and thesensor circuit can be set to monitor changes exceeding a predeterminedthreshold value. An advantage of optically transmissive sensor wire 38'over fusible wire 38 is that it does not require replacement after anovertemperature condition has been corrected. This same advantage can,of course, be provided by using a conductive material for wire 38 thatchanges resistance with temperature, rather than melting.

In operation, each run of the electrical system is protected by acoextensive length of sensor line or wire whose characteristics changewith temperature. Those characteristics are monitored by controlcircuitry which shuts off power to the associated run when anovertemperature condition is detected. Thus, when lines 12, 13 or wires31, 32 present a hazard due to increased temperature, but are not in anamperage overload, short or ground fault condition, the inventionprovides protection to shut power off when the usual protective deviceswould not do so.

Those skilled in the art to which the invention relates will appreciatethat other substitutions and modifications can be made to the describedembodiments without departing from the spirit and scope of the inventionas described by the claims below.

What is claimed is:
 1. In an electrical service including first lengthsof hot, neutral and ground lines connected between a utility servicepower hookup and a main breaker; second lengths of hot, neutral andground lines connected between the main breaker and a plurality ofbranch circuit breakers; and a plurality of third lengths of hot,neutral and ground wires connected between respective ones of theplurality of branch circuit breakers and respective ones of a pluralityof branch circuit electrical elements; the improvement comprising:atleast one of a plurality of first lengths of sensor wires having sourceends and load ends and running respectively coextensively with acorresponding one of each of the plurality of third lengths of hot,neutral and ground wires; the sensor wire lengths being insulated fromthe third lengths of hot, neutral and ground wires except at sensor wiresource and load ends; the sensor wire load ends being connected torespective ones of the third lengths of ground wires at end-of-the-runones of the electrical elements; and means connecting the sensor wiresource ends to corresponding ones of the branch circuit breakers fortripping the corresponding branch circuit breaker when the connectionbetween the associated sensor wire load end and length of ground wire isinterrupted.
 2. The improvement defined in claim 1, furthercomprising:at least one of a plurality of second lengths of sensor linehaving source and load ends and running coextensively with the secondlengths of hot, neutral and ground wires; the second sensor line lengthsbeing insulated from the second lengths of hot, neutral and ground linesexcept at first sensor line lengths source and load ends; the secondsensor line lengths load end being connected to the second length ofground line at the main breaker; and means connecting the second sensorline lengths source end to the main breaker for tripping the mainbreaker when the connection of the second sensor line lengths load endto the second length of ground line is interrupted.
 3. The improvementdefined in claim 2, further comprising:a second length of sensor linerunning coextensively with the first lengths of hot, neutral-and groundwires; the second sensor line length being insulated from the firstlengths of hot, neutral and ground lines except at second sensor linesource and load ends; the second sensor line load end being connected tothe ground line at the utility service power hookup; and meansconnecting the second sensor line source end to the utility servicepower hookup for tripping the main breaker when the connection of thesecond sensor line load end to the first length of ground line isinterrupted.
 4. The improvement defined in claim 1, wherein the branchcircuit breakers are designed to trip upon flow of predefined amperagevalues of current flowing through the corresponding lengths of hot andneutral wires, and the lengths of sensor wire comprise lengths ofconductive material which will fuse at a temperature less than atemperature of the hot and neutral wires when the predefined amperagevalue flows through them.
 5. The improvement defined in claim 1, whereinthe end-of-the-run electrical element of at least one of the branchcircuits is a receptacle outlet having a female ground prong connection;and further comprising an electrical device having a housing and a cordincluding a plug with a male ground prong mated with the female groundprong connection; the cord including lengths of hot, neutral and groundcord wires extending between the plug and the housing for providingelectrical power to the device; and further including a length of cordsensor wire running coextensively with the hot, neutral and ground cordwires; the cord sensor wire being insulated from the lengths of hot,neutral and ground cord wires except at cord sensor wire source and loadends; and the cord sensor wire load end being connected to the cordground wire at the housing; andmeans respectively connecting the cordsensor wire source end and length of cord ground wire to thecorresponding branch circuit sensor wire load end and length of branchcircuit ground wire, and disconnecting the connection between thecorresponding branch circuit sensor wire load end and the correspondingbranch circuit ground wire in response to the insertion of the pronginto the prong connection.
 6. The improvement defined in claim 5,wherein each male and female ground prong includes electrically isolatedhalves, a first one of which is connected to the length of cord groundwire and a second one of which is connected to the source end of thelength of cord sensor wire; and the prong connection is split intoelectrically isolated halves, a corresponding one of which is connectedto the corresponding length of branch circuit ground wire and a secondone of which is connected to the load end of the corresponding length ofbranch circuit sensor wire.
 7. The improvement defined in claim 1,wherein the lengths of hot, neutral, ground and sensor wires of each runare embedded in a common cabling.
 8. In an electrical service includingfirst lengths of hot, neutral and ground lines connected between autility service power hookup and a main breaker; second lengths of hot,neutral and ground lines connected between the main breaker and aplurality of branch circuit breakers; and a plurality of lengths of hot,neutral and ground wires respectively connected between the plurality ofbranch circuit breakers and a plurality of branch circuit electricalelements; the improvement comprising:a plurality of lengths of lighttransmissive sensor paths running respectively coextensively with theplurality of hot, neutral and ground wires; means for generating a lightsignal at circuit end over of the electrical elements; the sensor pathshaving load ends connected to receive respective ones of the lightsignals from the light signal generating means, and having source ends;and means connecting the sensor path source ends to corresponding onesof the branch unit breakers for tripping the corresponding branchcircuit breaker when the optical signal received at a correspondingsensor path load end is interrupted.